under IRC REG 1.1212-1 only 1256 contract losses can be carried back and you need to have made the loss carryback election with both your 2020 and 2021 returns. I cannot find anything as to whether or not late elections are allowed.
if you can or have made the elections 2020 would be carried back to 2017 and then forward. then 2021 to the extent available would be carried back to 2018 then forward. those losses get reported on form 6781 to allow the 60/40 split.
here are the rules as to how much can be carried back and forward FROM FORM 6781 INSTRUCTIONS
Box D. Net Section 1256 Contracts Loss Election
If you have a net section 1256 contracts loss you can elect to carry it back 3 years. Corporations, partnerships, estates, and trusts are not eligible to make this election. Your net section 1256 contracts loss is the smaller of:
• The excess of your losses from section 1256 contracts over the total of (a) your gains from section 1256 contracts plus (b) $3,000 ($1,500 if married filing separately), or
• The total you would figure as your short-term and long-term capital loss carryovers to 2024 if line 6 of Form 6781 were zero. Use a separate Schedule D (Form 1040) and Capital Loss Carryover Worksheet (in Pub. 550) to figure this amount.
The amount you can carry back to any prior year is limited to the smaller of:
• The gain, if any, that you would report on line 16 of Schedule D (Form 1040) for that carryback year if only gains and losses from section 1256 contracts were taken into account; or
• The gain, if any, reported on line 16 of Schedule D (Form 1040) for that carryback year.
The amounts just described are figured prior to any carryback from the loss year. Also, the carryback is allowed only to the extent it doesn’t increase or produce a net operating loss for the carryback year. The loss is carried to the earliest year first.
Make the election by checking box D and entering the amount to be carried back on line 6. To carry your loss back, file Form 1045 or 1040-X. Attach an amended Form 6781 and an amended Schedule D (Form 1040) for the applicable years.
On the amended Forms 6781 for the years to which the loss is carried back, report the carryback on line 1 of that year’s amended Form 6781. Enter “Net section 1256 contracts loss carried back from” and the tax year in column (a), and enter the amount of the loss carried back in column (b).
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Turbotax is not available for 2017-2019 unless you used the desktop version and still have it installed. Reinstalling won't work because no updates or states are available for those years.
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I strongly suggest you do 2020 c/b c/f filings and wait until they're approved before doing 2021.
